1. Field
This invention is directed to the process of purifying raw juice extracted from sugar beets prior to crystallization of the sucrose contained in the juice.
2. State of the Art
In the conventional production of crystallized sucrose (sugar) from sugar beets, a "raw juice" is initially obtained by diffusion of soluble material from the beets. The raw juice is then partially purified. The purpose of this initial purification step is to remove a significant portion of the "non-sucrose" fraction from the juice. The partially purified juice exhibits improved subsequent processing, yields a higher recovery of crystallized product and improves product quality with respect to color, odor, taste and solution turbidity.
The most commonly used method for raw beet juice purification is ubiquitous, and is based upon the addition of lime and carbon dioxide. The initial steps of this method occur prior to crystallization, during a phase commonly referred to as the "beet end" of the process. The sugar beets are typically diffused with hot water to extract a "raw juice" or "diffusion juice". The raw juice contains (1) sucrose (2) nonsucroses and (3) water. The term "nonsucroses" includes all of the sugar beet derived substances, including both dissolved and undissolved solids, other than sucrose, in the juice. Other constituents which may be present in the raw juice are not of concern to the present invention.
The raw juice is heated to high temperature, and a solution/suspension of calcium oxide and water (milk of lime) is added to the juice. The juice is then treated with carbon dioxide gas to precipitate the calcium oxide as calcium carbonate. This step is commonly called "first carbonation" and it is the foundation of the conventional purification scheme, resulting in a "first carbonation juice." During this step, various nonsucrose compounds, color etc. are removed or transformed by reaction with the lime or by absorption by the calcium carbonate precipitate.
Conventionally, the calcium oxide and the carbon dioxide are produced by heating limerock (calcium carbonate) in a high temperature kiln. The calcium carbonate decomposes to calcium oxide and carbon dioxide, which are then recombined in the first carbonation step. The resulting calcium carbonate "mud" is usually removed from the first carbonation juice by settling clarifiers or by appropriate falters. The resulting "lime waste" is difficult to dispose of and contains about 20-30 percent of the original raw juice non sucrose. The first carbonation juice is most commonly sent to a second carbon dioxide gassing tank (without lime addition). This gassing step is often referred to as "second carbonation." The purpose of the second carbonation step is to reduce the level of calcium present in the treated ("second carbonation") juice by precipitating the calcium ions as insoluble calcium carbonate. The calcium precipitates, often called "limesalts," can form a noxious scale in downstream equipment, such as evaporators. The second carbonation juice is usually filtered to remove the precipitated calcium carbonate.
Following these purification steps, the remaining juice is referred to as "thin juice". Only about 20-30 percent of the nonsucroses in the raw juice are susceptible to removal by liming and carbonation treatments. The remaining nonsucroses ("non-removable nonsucroses") have chemical characteristics which make it impossible to remove them through those expedients. These constituents remain in the thin juice.
The thin juice, which may range typically from about 10 to about 16 percent solids, based upon the weight of the juice, is sent to a concentration step to raise the solids content to about 60 to about 70 percent by weight. There results a purified syrup, which is referred to as "thick juice."
A number of variations to the liming and carbonation process are in use in the industry. Typical alterations to the basic process described include:
"Preliming," which is conducted prior to first carbonation. This procedure involves the progressive controlled addition of lime, whereby certain nonsucroses are precipitated and stabilized for subsequent removal.
"Main liming," which is also conducted prior to first carbonation, and involves the addition of lime to the juice at conditions of high pH and high temperature, whereby to destroy certain nonsucroses and to produce a more thermostable juice.
"Defeco-carbonation," which involves the simultaneous addition of lime and carbon dioxide on a continuous basis.
"Adjustable processes," wherein the amount of lime, the temperature, and the addition of other chemicals, such as sulfur dioxide or soda ash, are incorporated into the liming and carbonation process to make the process more adaptable to changing conditions.
During the crystallization process, the purified thick juice produced on the beet end is sent to the "sugar end." The function of the sugar end of the process is to crystallize the sucrose from the thick juice as a marketable product. This product is most commonly referred to as "sugar" by consumers or others outside the industry. It is not feasible to crystallize all of the sucrose in the thick juice as acceptable product. A large amount of this sucrose is lost to a discard called "molasses". This inefficiency is largely due to the reality that the liming and carbonation "purification" procedures actually remove only a minor portion of the nonsucrose in the juice. The presence of residual nonsucrose in the thick juice significantly interferes with the efficient crystallization and recovery of the sucrose because of inherent crystallization and solubility effects. Consequently, a low value molasses is an unavoidable byproduct of the crystallization procedure.
The molasses recovered from the crystallizers contains substantially all the nonsucrose components originally in the thick juice, together with a significant portion, typically on the order of about 15 percent, of the original thick juice sucrose content. The molasses thus represents the major loss of sucrose in a beet factory. It is usually discarded as an animal feed. Occasionally, specialized processes are employed to recover additional sucrose from this byproduct.
The typical beet sugar crystallization process consists of three crystallization procedures operated in series. These crystallization steps are often referred to as "A," "B" and "C" crystallizations, respectively; where "A" corresponds to "white;" "B" corresponds to "high raw" and "C" corresponds to "low raw" crystallizations, respectively, according to an alternative terminology.
Each subsequent crystallization step receives the mother liquor from the preceding step. The mother liquor from the last crystallization step is discarded from the process as molasses. Each crystallization step removes sucrose. Accordingly, the mother liquor increases in nonsucrose concentration with each succeeding step. The decreasing purity of the mother liquors interferes progressively with the rate of crystallization and the quality of the crystallized product from the B and C steps. The crystallization rate is typically an order of magnitude lower during the C crystallization step than during the A crystallization step. Crystallized product from the B and C steps is generally of such poor quality that it is recycled to the A crystallization step. Generally, only sucrose crystallized in the A step is considered to be of marketable quality.
Methods have been proposed for the production of marketable sucrose that avoid liming and carbonation purification procedures. These proposals have not been adopted because of their significant shortcomings.
Raw juice crystallization without purification has been proposed. However, the sucrose crystallized directly from raw juice exhibits very high color, off-odor, off-taste and high suspended solids content. Significant evaporator scaling occurs. Recovery of product sucrose is also extremely poor because the mother liquor produced by crystallizing unpurified raw juice inevitably carries even more sucrose to the molasses byproduct.
Ion exchange purification of process juices has been proposed. Such a method depends upon exchanging juice nonsucroses for less noxious materials. Because very large mounts of the exchangeable nonsucroses are present in raw juice, at least equal amounts of less noxious nonsucroses must be provided for the exchange. Consequently, these methods have been considered generally impractical because of the very large mounts of expensive regenerants and regenerant waste produced. In addition, lime-removable nonsucroses are more cheaply removed with lime than with ion exchange. Various methods and equipment used for purifying raw sugar juice by ion exchange are disclosed in British Patent No. 1,043,102, U.S. Pat. Nos. 3,618,589, 3,785,863, 4,140,541 and 4,331,483.
Membrane filtration has been proposed, and methods have been tested involving the use of membranes to separate materials of differing molecular weight from the raw juice. Such methods necessitate a high capital cost, have a short membrane lifetime with expensive replacement cost and significant loss of sugar to the "concentrate" membrane byproduct stream. The resulting juice is not of a higher purity than that realized with conventional liming and carbonation. Suggestions have been made to combine the membrane purification with liming and carbonation, electrodialysis or ion exchange demineralization. A proposed method of purification of raw sugar juice involving membrane ultrafiltration is disclosed in U.S. Pat. No. 4,432,806.
Chromatography has been proposed, and is presently used in a few locations, as a method of separation applied to the byproduct molasses. The purpose of molasses chromatography is to recover residual sucrose from this byproduct stream. The resulting chromatographic product is of relatively poor quality due to high color, odor and low purity. It is generally mixed with conventionally produced syrups to lessen its detrimental effects upon crystallization. The product is sometimes cleaned-up first with ion-exchange or it is returned to the beet end for a second pass through the liming and carbonation purification process.
Because the molasses chromatographic separator is designed to operate on the final byproduct stream of the sugar factory, it has no beneficial impact on the upstream purification or crystallization processes. Its purpose is to act as a last step to recover the sucrose lost in the molasses. It is designated to operate on material derived from the conventional liming and carbonation process.
A method and apparatus for chromatographic molasses separation are disclosed in U.S. Pat. No. 4,312,678. Other methods and apparatus using simulated moving bed chromatographic separators are disclosed in U.S. Pat. Nos. 2,985,589, 4,182,633, 4,412,866 and 5,102,553.